Friday 10 February 2012 06.00 EST
First published on Friday 10 February 2012 06.00 EST

John F Kennedy, always a US icon, has over the years acquired a life story that's almost all sex and violence. Assassinated on 22 November 1963 in an atrocious public death, JFK and his record have become progressively tarnished by the sexual secrets of Camelot.

The names of Judith Campbell Exner, Mary Pinchot Meyer, Gunilla von Post, Marlene Dietrich and two secretaries dubbed "Fiddle" and "Faddle" are now associated with the 35th president's private life as much as Lee Harvey Oswald and Jack Ruby are with his violent death.

Marion ("Mimi") Beardsley Fahnestock Alford is the latest notch to be carved into the presidential bedpost. She was first outed by Robert Dallek in his 2003 JFK muckraker, An Unfinished Life, as a "tall, slender, beautiful" 19-year-old college sophomore with the pet-name "Monkey", and endured a firestorm of post-Lewinsky media intrusion. Now, as Mrs Alford, a sixtysomething divorcee, she has decided to take control of "my story".

Actually, Once Upon a Secret is less an act of independent self-possession, more the helpless revelation of a woman as a victim. Her carefully constructed memoir, despite its marketing, is not so much a saucy kiss'n'tell of hanky panky in the White House, rather a tragic three-act case study of a young woman who flew too close to the sun.

In American class terms, Mimi is medium posh. She describes a childhood of "preppie privilege", growing up "in a rambling colonial farmhouse" in New Jersey. Her parents were classic east coast Wasps, but no picnic: her father a manic depressive; her mother a domestic diva. Reading between the lines of her tight-lipped family history, it's clear that, as a young girl, Mimi was stifled, obedient, anxious – and low on self-esteem. "Everyone we knew was a Republican," she writes, "and shared the same Protestant faith."

In high school, Mimi says she had "a run of bad luck" with boys. When her luck changed and she landed a suitor in eighth grade, she let him kiss her, once. Even in the late 1950s, this was not exactly the primrose path of dalliance. "That was the last kiss anyone bestowed on me through high school," she writes. "Monkey" Beardsley was a psychosexual accident waiting to happen.

The first sign of trouble, aged 17 and feeling "like I didn't belong", was anorexia, though no one was using the word then. By 1962, barely 19, Miss "Changed Most Since Sophomore Year" was a young woman who, in her own words, "could talk and flirt and parry [with boys] easily. I just needed to find someone who understood me."

It was at the climax of this first act in her life that, exploiting a school connection, young Marion Beardsley wrote to the first lady, Jackie Kennedy, and landed a job as a White House intern. Rarely has a naive virgin stepped into a more perilous scenario.

Alford says that "the word feminism had not yet entered my vocabulary". It's a moot point whether women's lib could have inoculated this vulnerable 19-year-old against the aphrodisiac of supreme power. It was as if, she writes, on the brink of her fall, "I had been awarded membership in an elite club without having to go through the initiation process".

Almost, but not quite. She was already in too deep. On only her fourth day at 1300 Pennsylvania Avenue, Alford found herself in the White House swimming pool with "Fiddle" , "Faddle", JFK and his procuring "first friend", Dave Powers. Cocktails in the president's suite followed. According to Alford, the president "couldn't resist a girl with a little bit of social register in her". Late in June 1962 Mimi Alford experienced "the thrill of being desired". Cruelly, she "cannot describe what happened that night as making love". But she resists any charge of date rape. "I wouldn't call it non‑consensual, either."

The 18-month affair Alford reveals reduced her 19-year-old self to the status of presidential plaything. She would do her college classwork in the limo on the way to have sex. JFK never kissed her on the mouth. Even in bed, she called him Mr President. Afterwards, she would listen to Little Peggy March or the Shirelles ("Will You Love Me Tomorrow?"). He preferred Tony Bennett or Frank Sinatra.

The dark side of the man she calls "the Great Compartmentaliser", and who would identify himself on the telephone as "Michael Carter", was never far away. One day in the swimming pool, he decided that Dave Powers was looking "tense", and coerced Alford into giving the first friend a blow job. "I don't think the president thought I'd do it, but I'm ashamed to say that I did. The president silently watched." With sex, came drugs. Alford claims she was "the guinea pig" for the president's fascination with amyl nitrate – poppers.

The exercise of power can be very discreet. The secret service turned a blind eye. Alford kept her shame to herself, and would do so for more than 40 years: "Blinded by the president's power and charisma, I was fully committed to keeping our affair secret." It was a joyless business. "I can't say our relationship was romantic. It was sexual, it was intimate, it was passionate," she writes. "But there was always a layer of reserve."

Don't look to Once Upon a Secret for much new insight into JFK's presidency. Alford tells us that during the Cuban missile crisis, her lover confided "I'd rather my children be red than dead", but little else. On the death of his baby son, Patrick, he shared condolence letters with Alford, "tears rolling down his cheeks". That was probably the closest she came to the Great Compartmentaliser's heart.

Then she met a boy her own age, Tony Fahnestock, and got engaged. She continued to see the president. In the third act of this romantic tragedy, it's only on JFK's assassination that she confesses all to her future husband. Fahnestock, with terrible cruelty, says he will forgive and marry her, but that she must never tell a soul.

The burden of this secret (which she gradually shares with a tiny circle of girlfriends) stifles her emotional life, poisons her marriage, and traps her in what she calls "her emotional shell". Because this is America, where stories must have happy (or at least, feel-good) endings, she has therapy, finally meets Mr Right, and is able to "let go of my secret, and share". Sadly, for her, it may be too late. At the end of Once Upon a Secret she confesses she has perhaps "never been part of the story" and was only "a footnote to a footnote".

• This correction was published on 19 February:The White House is at 1600 Pennsylvania Avenue, Washington DC, not 1300, as we said in "Ask what you can do for JFK…" (Books).